The present invention concerns an electro-optical cell, in particular a liquid crystal cell, or an improved electrochemical photovoltaic cell, in particular for a timepiece, and more particularly connecting means intended to establish an electric connection between the electrodes of the cell and an electric supply or control circuit. The present invention also concerns a manufacturing method for the aforementioned cell.
A first known category of photovoltaic cells converts light into electricity by exploiting the photovoltaic effect which appears at the junction of semi-conductors. The semi-conductor material fulfils at the same time the functions of light absorption and separation of the resulting electric charges (electrons and holes). As the material must have a high level of purity and be free of defects, failing which the electrons and holes would be combined again before having been able to be separated, the manufacturing costs are high.
The present invention is intended for a second type of photovoltaic cell called an electrochemical cell, a schematic diagram of which is shown in FIG. 1 annexed to this application.
Such a cell, designated as a whole by the general numerical reference 1, includes a first transparent front strip 2 and a second rear strip 4 which is also transparent. These two strips 2 and 4 each include, on their facing faces, a first electrode 6 also called a counter-electrode, and a second electrode 8 also called a photoelectrode. These electrodes 6 and 8 are usually made in the form of a thin layer of an indium/tin or tin oxide/antimony mixture.
Strip 4 further includes a layer 10 made in the form of a porous structure 12 having a large effective surface formed of particles of an active semi-conductor oxide such as, for example, titanium dioxide TiO2, the role of which will be described hereinafter.
Strips 2 and 4 are joined to each other by a sealing frame 14 which extends along, the perimeter thereof and which defines a volume 16 filled with an electrolyte 18 containing a mediator such as the redox couple iodide-tri-iodide Ixe2x88x92/I3xe2x88x92.
Titanium oxide TiO2 which constitutes layer 10 is a semi-conductor which is normally not sensitive to visible light because of the width of its forbidden band, and which only begins to absorb close to ultraviolet. It can nonetheless be sensitised by adsorption of a colouring agent such as a complex compound of a transition metal, in particular ruthenium or osmium, which allows an incident photon/electron conversion rate approaching the unit. The light absorption, symbolised by arrows in FIG. 1, is assured by the colouring agent chemically adsorbed at the surface of semi-conductive material layer 10. After having been excited by the absorption of a light photon, the colouring agent can transfer an electron into the conduction band of the semi-conductor of layer 10. The electric field present within the semi-conductive layer 10 allows the extraction of this electron. After transferring the electron, the colouring agent returns to the fundamental oxidised state. The recombination between the electron in the conduction band and the hole on the oxidised colouring agent is much slower than the reduction of the oxidised colouring agent by the mediator. Consequently, the charge separation is efficient. The oxidised mediator is reduced to counter-electrode 6.
The present invention is also intended for so-called electro-optical display cells, in particular liquid crystal cells which, like electrochemical photovoltaic cells, include.
a first transparent front strip whose upper surface constitutes the front face of said cell;
a second rear strip whose lower surface constitutes the rear face of said cell;
each of the strips including on their facing faces at least one electrode, these electrodes being intended to be connected to a display control circuit and partially coming level with the edge of the corresponding strip to define an electric contact zone in distinct locations on the cell perimeter;
the strips being separated by a sealing frame defining a volume for the confinement of an optically active medium, and
connecting means for establishing the electric connection between each electrode and said display control circuit.
A constant problem in the field of electrochemical cells of the kind described hereinbefore lies in the connecting means to be used for establishing a reliable electric contact between the electrodes of the cell and an electric supply circuit. This problem is also encountered with electro-optical display cells wherein the electrodes of the cell have to be connected to a device for generating electric control signals which modify the electro-optical characteristics of the material confined between the strips of the cell.
The Applicant has already provided a response to this problem by proposing, in his French Patent No. FR-B1-2 637 110, an improved electro-optical cell wherein each of the two cell strips carries a transparent electrode over its entire surface, this electrode partially coming level with the edge of the strip onto which it has been deposited to define a lateral electric contact zone. In this Patent, the connecting means include a contact pad made of an adhesive synthetic material charged with conductive particles which are added onto the edge of each strip, to the right of the contact zone, and which assure the electric connection with the electric control circuit of the cell.
During use, this connection technique has nonetheless exhibited several drawbacks. On the one hand, the strips of the cell must be staggered in relation to each other, in order to provide access to the electrodes and to make the electric connections. Such an arrangement makes series manufacturing of the cells difficult, in particular when the cells are circular, and requires additional time-consuming operations.
On the other hand, as recalled hereinbefore, the connection between the electrodes and the control circuit is made via the electric contact zone where each of the electrodes of the cell comes level with the edge of the strip onto which it has been deposited. The thickness of this contact zone is very small, typically of the order of a thousand angstroms. Problems of electric contact or even breakage of the mechanical connection between the electrodes and the contact pads thus appear.
Moreover, timepieces such as wristwatches are known wherein an electrochemical photovoltaic cell of the aforementioned type is conventionally arranged under the crystal so as to be sufficiently exposed to light. By conversion of the visible light into an electric current, the photovoltaic cell powers, via an electric circuit, the horometric means of the watch.
Arranged underneath the watch crystal, the photovoltaic cell covers the time indicators (minute and hour hands, etc.) as well as the dial of the watch. Nonetheless, since this cell is quasi transparent, the indicators and the dial remain visible through the cell.
The electric connections of an electrochemical cell, arranged in two distinct locations on the cell perimeter, must however be masked in order to made them invisible from the exterior when one looks through the watch crystal. In order to overcome this problem, the only known solution is to increase the size of the bezel to mask the electrodes, and thus to reduce the useful surface of the crystal and the dial, which is detrimental to the aesthetic appearance of the watch and to the ability to read the information displayed on the dial. Another solution consists in reducing the dimensions of the photovoltaic cell, which is detrimental to its efficiency.
Another solution is known from the unexamined Japanese Patent Application No. 61-201289 which concerns a liquid crystal cell including a first front transparent plate 1a, and a second back plate 1b which is also transparent. These two plates 1a and 1b each include on their opposite faces a set of electrodes, respectively 2a and 2b, said electrodes being flush with the edge of the plate on which they have been deposited to define electric contact zones. Spacers such as micro-balls or glass fibres are dispersed between the two plates 1a, 1b in order to keep the thickness of the liquid crystal layer uniform over the entire surface of the cell. A connector is made by means of a flexible insulating film 3a on which is printed an electrically conductive ink 4 having adhesive properties. Ink 4 is printed according to substantially rectilinear strips which have the same pitch as the transparent electrodes which are flush along the edges of the cell. Between the conductive strips, an insulating ink with adhesive properties is printed, so that the printed surface becomes plane. Connection film 3a is then secured via pressure against the edges of the cell to form the sealing thereof. During this operation, electrically conductive ink 4 infiltrates slightly between plates 1a, 1b of the cell, so that the electric connection is established between the electrodes of the cell and the conductive strips. After flexible connecting film 3a has been applied against the edges of the cell, liquid crystal 5 is injected in the gap between the two glass plates 1a and 1b, either through a filling opening formed in one of plates 1a or 1b, or at a location where flexible connecting film 3a has not yet been secured against the edge of the cell. Afterwards, the filling opening is sealed in order to obtain a liquid crystal cell with connecting film 3a secured against the edge of said cell.
The cell described above has the following drawbacks. It should be noted, on the one hand, that the conductive ink is directly in contact with the liquid crystals, thereby creating a non-negligible risk of contaminating said crystals and damaging the performance of the cell. On the other hand, such a method does not allow the cells to be manufactured in batches. Indeed, in the case of a conventional liquid crystal cell, the sealing frame allows the two substrates of the cell to be secured to each other, and the cell to be handled easily. In particular, the operation of filling the cells with the liquid crystal can be performed when the cells are still in a strip, i.e. not individually separated. However, in the case of the aforementioned Japanese Patent Application, the two plates which are not secured to each other have to be held in place with respect to each other until the connecting film has been secured to the edges of said Mates. Such an operation can thus only be performed individually on each part.
The present invention therefore concerns an electro-optical cell, in particular a liquid crystal cell, or an electrochemical photovoltaic cell including:
a first transparent front strip whose upper surface constitutes the front face of said cell;
a second strip which may or may not be transparent, and whose lower surface constitutes the rear face of said cell;
the strips each including, on their facing faces, at least one electrode, these electrodes being intended to be connected to an electric supply or control circuit and coming partially level with the edge of the corresponding strip to define a lateral electric contact zone in distinct locations on the cell periphery;
the strips being separated by a sealing frame defining a volume for the confinement of a photoelectrically or electrooptically active medium; and
electric connecting means for establishing the electric connection between each electrode and said supply or control circuit, characterised in that the connecting means include a contact pad made of an electrically conductive material added onto each electrode in the lateral contact zone where the latter is level with the edge of the strip onto which it has been deposited, so as to fill the volume of the cavity defined by each electrode with the strip which is opposite thereto, in order to increase the surface of the lateral contact zone.
As a result of the features of the present invention, it is possible to increase the surface of the lateral electric contact zone via which each electrode of the cell is connected to the electric supply or control circuit. The electric connection between the electrodes and the electric supply or control circuit is thus much more reliable.
According to another advantageous feature of the invention, the electric connecting means of the photovoltaic or electrochemical cell include a contact part affixed to the right of the electric contact zone where each electrode comes level with the strip on which it has been deposited, this electric contact part realising the electric contact transfer on one of the front or back faces of the cell.
It is thus possible, for example, to make an electrochemical photovoltaic cell whose connecting means, which are intended to establish the electric connection between the cell electrodes and an electric supply circuit, are transferred on the back face of said cell. The cell sealing frame and the contact transfer on the back face of the latter are thus superposed rather than juxtaposed laterally as is the case in the prior art. The bezel of the timepiece or any other means intended to mask the sealing frame of the cell also masks the electric contact zones thereof. The dimensions of the crystal, and thus of the timepiece dial can be increased, which is advantageous as regards the aesthetic appearance of the watch and the ease with which the information on the dial thereof can be read. Likewise, the photovoltaic cell can be larger, which increases its efficiency. Finally, it is no longer necessary to stagger the strips of the cell. The space requirement of the cell is therefore reduced, which facilitates the mounting thereof in particular in a timepiece. The manufacture of such cells can also be performed in batches, allowing the cost price thereof to be reduced as a result of large scale manufacture.
According to another aspect, the present invention also concerns a manufacturing method for the aforementioned cell wherein, prior to separation of the cells, each of the cell electrodes is extended slightly beyond the final geometrical limit of said cell by means of a deposition of an electrically conductive material.
According to an aspect of the method of the invention, the lateral electric contact zones are metallised by vacuum evaporation or covered with a layer of an electrically conductive material at the locations where the electric contact parts will subsequently be applied.
Another object of the invention is to provide a timepiece including an electrochemical photovoltaic cell according to the invention intended to supply, via an electric supply circuit, in particular the horometric means of the timepiece and/or an electro-optical display cell for the presentation of information.